Please help me find my garage door opener. I'm not sure if my BMWz has one or not.

I believe that your bimmer might have the unviersal garge door opener as long as your Z4 has a the SOS wirless system which comes with the Universal Garge door opener. if you have this it would be on the bottom of your rearview mirror.

More dollars than sense

As far as I know, cars don't normally come with garage door openers. If they did, anyone can drive up your driveway and enter your garage (unless the openers can be programmed for each garage)!
The door opener normally comes as a seperate remote-like device for the garage.